Hitherto, glass is widely and largely used as an optical material since it has a variety of refractive indexes and dispersion values. However, glass has several drawbacks. For example, the refractive index of glass is large and any glass with a low refractive index and large dispersion has not been produced. Since glass has poor processability, the production of an unsymmetric lens costs much. Glass has large specific gravity. In addition, glass is hard and fragile.
As an economical optical material with good processability, there are known plastics such as polymethyl methacrylate and polystyrene. However, the number of practically useful plastic optical materials is not large. Further, the plastic optical material has poor heat resistance and/or flexibility. It is not easy to produce an article of a complicated shape from the plastic optical material.
A cross-linked optical siloxane polymer is an attractive optical material, since it has not only good processability but also good transparency and heat resistance, and rubbery elasticity and after cured.
U.S. Pat. No. 3,228,741 (issued on Jan. 11, 1966) discloses a gas permeable contact lens made of a transparent silicone rubber.
U.S. Pat. No. 3,996,189 (issued on Dec. 7, 1976) discloses a method for matching a refractive indexes of a silica filler and of a siloxane polymer with each other, in which a suitable amount of a siloxane polymer containing phenyl groups so as to match the refractive index of the siloxane polymer with that of the silica filler to produce an optically transparent siloxane polymer composition. In this method, a siloxane polymer having vinyl groups at both chain ends and 6 to 16% by mole of the phenyl groups and a siloxane polymer having silicon hydride groups (.tbd.SiH) at both ends are subjected to an addition reaction in the presence of a platinum catalyst, namely hydrosilyl reaction to give a transparent cross-linked siloxane polymer.
Further, Japanese Patent Kokai Publication (unexamined) No. 130844/1980 (laid open to public inspection on Oct. 11, 1980) describes the use of a polymeric composition comprising a siloxane polymer containing phenyl groups blocked with vinyl groups at both chain ends and having a viscosity of 100 to 15,000 cP at 25.degree. C. with a molar ratio of methyl/phenyl of 1:1 to 10:1, a siloxane polymer containing a silicon hydride group and having a viscosity of 0.5 to 5,000 cP at 25.degree. C. and a platinum compound as a cladding material of an optical glass fiber.
U.S. Pat. No. 3,341,490 (issued on Oct. 12, 1967) discloses a transparent siloxane polymer composition comprising a blend of dimethylsiloxane/phenylmethylsiloxane copolymer and dimethylsiloxane/methylvinylsiloxane copolymer and a reinforcing silica filler.
In all of the conventional arts, the refractive index of the transparent siloxane polymer was modified by the addition of the siloxane polymer containing the phenyl groups.
One known method for modifying the refractive index of the siloxane polymer comprises homo- or co-polymerizing a cyclic siloxane monomer having a phenyl group (e.g. 1,3,5-trimethyl-1,3,5-triphenylcyclotrisiloxane, 1,3,5,7-tetramethyl-1,3,5,7-tetraphenylcyclotetrasiloxane, hexaphenylcyclotrisiloxane, octaphenylcyclotetrasiloxane and the like) and cross-linking the polymer with a polymeric material or a blend of a siloxane polymer such as dimethylsiloxane/diphenylsiloxane copolymer, dimethylsiloxane/methylphenylsiloxane and methylphenylsiloxane polymer to which vinyl groups and/or silicon hydride groups are introduced for cross-linking.
However, these conventional methods have several drawbacks such that:
1. Since the refractive index is modified by the content of the phenyl group, the refractive index can not be changed beyond a certain range.
2. Since preparation and purification of the siloxane monomer having the phenyl group is rather difficult, it is very expensive to produce a monomer which is pure enough to be used as an optical material.
3. Trial and error polymerization is required to precisely control the refractive index of the siloxane polymer by the use of the siloxane monomer having the phenyl group.
4. In order to provide several kinds of siloxane polymers having different refractive indexes, it is necessary to produce polymers with varying kinds and/or amounts of the comonomer to be copolymerized. This is troublesome in view of quality control and investory control.
5. When the refractive index of the siloxane polymer is modified by polymer blending, kinds of the siloxane polymers are limited by their compatibility. Even if the siloxane polymers having good compatibility are used, the blend has large Rayleigh scattering so that the transparency of the blend is deteriorated.
However, none of the conventional art intends to precisely control the refractive index of the siloxane polymer since the polymer is used, for example, as a contact lens. Nevertheless, the control of the refractive index of the siloxane polymer is important, since such control is basic technique for designing an optical wave guide, an optical fiber and a lens. In addition, to achieve good light transmission in an optical transmission system, refractive indexes of materials and/or elements should be matched. For this end, precise control of the refractive index of the siloxane polymer is highly desired.